CROSS-REFERENCE TO RELATED APPLICATIONS
The invention relates to syringes for dosed injection of a medicine from an exchangeable cartridge of the kind having a piston which is forced into a tubular cartridge to press out a dose of medicine corresponding to the movement of the piston, the syringe having a housing comprising a cartridge holder and a dosing mechanism by which a dose is set and subsequently injected by successively advancing a piston rod to press the piston into the cartridge, said dosing mechanism comprising a threaded spindle and a nut member co-operating with the spindle so that by setting of a dose relative rotation of the spindle and the nut member will move the nut member along the spindle, the position of the nut member on the spindle defining how far the piston rod is advanced during the injection, and the relative rotation of the spindle and said nut member being locked to keep the nut member in its position on the spindle when the dosing mechanism is not operated to set a dose.
To make a syringe as easy to handle as possible, the number of operation means should be minimized. Ideally the operation means only comprises a means for setting a dose and a means for injecting the set dose. Disposable syringes only comprising these two kinds of means are known, but if it is wanted to have a syringe which does not infer that the dosing mechanism is disposed of each time a cartridge integrated in such a pen is empty, the cartridge must be exchangeable. This infers that one more operation must be performed frequently as the cartridge must be changed when it is empty.
This in itself should cause no more complications than the fact that a new disposable syringe must be taken into use when the previous one is empty. However the syringe must be screwed apart to open access to the used cartridge and screwed together again when a new cartridge has been inserted. This is a mounting function by which care must be taken not to damage the threads by which the syringe is screwed apart and the screwing movement is difficult to perform when the tactile motor function is reduced.
Further, the medicine is pressed out of the cartridge by a piston which is pressed successively further into the tubular cartridge by the piston rod forming a part and the dosing mechanism. When the cartridge is empty said piston rod projects into the cartridge in almost the total length thereof. To change the cartridge the piston rod must first be drawn out of the empty cartridge, and thereafter it must be brought back to its initial position in the dose setting part. The last operation is made possible by locks getting unlocked when the empty cartridge is removed from the syringe, whereafter the piston rod may be pushed or screwed back to its initial position.
Although this retraction of the piston rod may seem to be simple, it infers a major risk for destroying the pen due to wrong handling as an attempt to press a piston rod which should have been screwed is just as destroying as screwing a piston rod which should have been pressed.